Localization of SP receptor and SP immunocytochemistry were conducted for the whole rat embryo of 14, 17, and 20 days of gestation and the new born on the day of birth. Both CNS and peripheral system was well developed as early as 17 day of gestation. Based on the membrane binding studies with 3HSP, the number of binding sites increased during the development till the first 4 weeks after birth when the brain size reached maturity, it decreased afterward. Purified SP membrane receptor from rat embryo brain was subjected for dissociation of subunit without enzymatic cleavage and iodination with Bolten Hunter reagent. Two different brain areas of mature young rat (brain stem and midbrain plus olfactory bulb) were subjected to Percol gradient separation. Two fractions (#6 & 8) of synaptosome isolated from the two area exhibited distinctive difference in 3HSP binding this might indicate the possible functional and chemical difference of the two fractions. Antibody against SP antibody has been initiated on New Zealand white rabbits.